What is the age of Marlin 3030 model 30A serial 20195318?

The ser.# you gave is a higher number and is best answered by Marlin Manufacturing, so you need to contact them at their address. Unless you can find someone that has a bluebook on Ser# and corrisponding years it's hard to give a value. However the gun was made 1984 to date, and depending on how long you have had it and the condition, it is worth $225 to $265 dollars in real good condition. Most sell for $150 to $200 without a scope. But I do agree with the valuation.